Add an alias called old_index to the new_index along with any aliases
returned in step 3, using the update aliases API.

In the future, we plan to change the upgrade API to perform a reindex-in-
place. In other words, it would reindex data from old_index to .old_index
then atomically delete old_index and rename .old_index to old_index.

If true, only very old segments (from a
previous Lucene major release) will be upgraded. While this will do
the minimal work to ensure the next major release of Elasticsearch can
read the segments, it’s dangerous because it can leave other very old
segments in sub-optimal formats. Defaults to false.

Use a GET request to monitor how much of an index is upgraded. This
can also be used prior to starting an upgrade to identify which
indices you want to upgrade at the same time.

The ancient byte values that are returned indicate total bytes of
segments whose version is extremely old (Lucene major version is
different from the current version), showing how much upgrading is
necessary when you run with only_ancient_segments=true.

The level of details in the upgrade status command can be controlled by
setting level parameter to cluster, index (default) or shard levels.
For example, you can run the upgrade status command with level=shard to
get detailed upgrade information of each individual shard.